1. Field of the Invention
The present invention relates to a signal transmission control system for controlling communication or data exchanges among a plurality of subscriber stations provided along signal transmission paths. In particular, the invention is intended to improve the signal transmission control system having dual transmission paths such that changing-over of the in-service transmission path to a stand-by transmission path upon occurrence of abnormal conditions can be realized smoothly and that the area suffering from such abnormality can be easily localized.
2. Description of the Prior Art
There has been widely used a signal transmission or communication system in which a number of subscriber stations are coupled to a set of transmission paths for performing information or data exchanges among the subscriber stations. In such communication system, a failure in the transmission path itself or in the sender portion of a subscriber station may exert adverse influence to the whole communication system, thereby rendering the data transmission impossible and possibly shutting down the whole system.
With a view to protecting the communication system from such disastrous situation, it is known and actually adopted to provide the transmission paths in duplicate with one of the transmission paths being used as the primary or in-service path, while the other is adapted to serve as the stand-by path for replacing the primary transmission path when some abnormality or failure occurs. More particularly, a plurality of transmitter and receiver stations which will be herein referred to also as subscriber stations are coupled to a pair of transmission paths which will be identified by labels A and B for convenience of description. Each of the subscriber stations is composed of a first input/output device connected to the transmission path A for data reception and transmission, a second input/output device connected to the other transmission path B for data reception and transmission, an arithmetic unit for executing arithmetic operations on the basis of the data or information received through the input/output devices and sending out the results of the operations to the transmission paths through the respective input/output devices, and a switching control unit for enabling the function of the first input/output device, when the transmission path is used as the primary or in-service path, and at the same time invalidating the output function of the second inpput/output device connected to the path B with only the input function thereof being validated. When the transmission paths A and B are interchanged the output function of the first input/output device is inhibited with the function of the second input/output device being wholly enabled. On the other hand, a transmission control station for controlling data transmission among the subscriber stations is separately provided for each of the transmission paths A and B with the control output function of the transmission control station for the transmission path B being inhibited. In such data transmission system, since only one transmission path is constantly being used, it is impossible to transmit simultaneously a number of data. Consequently, the transmission control station supervises and controls properly the subscriber stations between which the data exchange is to take place. With such arrangement of the data transmission control system, occurrence of abnormality in data or signal transmission is detected by the in-service transmission control station associated with the transmission path A which detection may be effected by utilizing the fact that access or call signal to the subscriber stations produced from the in-service control station is deviated from the standard signal, for example. When such abnormality is detected, the control station for the transmission path B is validated for operation while the control station for the transmission path A is caused to stop the supervisory operation. Correspondingly, when the call or access signal from the control station for the path B is supplied to the second input/output device of the subscriber station coupled to the path B, the switching control unit is operated, whereby the output function of the input/output devices of the subscriber stations connected to the path B is enabled with the output function of the input/output devices connected to the transmission path A inhibited. By virtue of such switching control for the transmission paths, it is certainly possible to perform the information transmission among the subscriber stations continuously without being interrupted or shut down.
However, in the hitherto known data transmission control system, it is impossible or at least very difficult to specify the place or location at which the abnormal situation or failure occurs, as a result of which extremely time-consumed procedures are required for restoring the disconnected subscriber station or transmission path to the original normal conditions. Thus, there is a need for the signal transmission control system which is capable of localizing the area or region at which the abnormal condition or failure takes place.